I am using iTextSharp to generate PDF documents, and save them to disk like this:
Document document = new Document(PageSize.LETTER, 50, 50, 80, 50);
Have you tried it without the FileAccess.ReadWrite, FileShare.ReadWrite properties?
I create PDF's using itextSharp everyday and have never ran into this issue - main difference between your code and mine is i don't have these.
I suspect the FileShare is leaving the document somewhat open, waiting for someone to finalise closing it with a save?